1. Field of the Invention
The present invention relates to surface polishing method and apparatus using a polishing plate or wheel which has a polishing face for polishing, lapping or grinding a surface of the workpiece in a plane.
2. Discussion of the Related Art
A surface polishing method and an apparatus for practicing the method are known for polishing, lapping or grinding a surface of the workpiece in a plane, such that the workpiece is held in sliding contact with a polishing face of a rotating polishing or lapping plate or wheel while the workpiece is rotated about its axis parallel to the axis of rotation of the polishing plate. The polishing face has a polishing surface, which has abrasive grains fixed or bonded thereto or on which loose or free abrasive grains are present. In this type of surface polishing method and apparatus, there is a general tendency that the polishing distance over which the abrasive grains are moved relative to the workpiece surface is different at different radial positions of the workpiece, in particular, at the peripheral and central portions (radially outer and inner portions) of the workpiece. This difference of the polishing distances at the different local radial positions of the workpiece surface increases with an increase in the size of the workpiece surface to be polished, lapped or ground, and is relatively large when the diameter of the circular workpiece surface or the length of a side of the rectangular workpiece surface is several tens of centimeters or more. The difference of the polishing distances causes a difference in the local polishing efficiency of the workpiece surface, which tends to cause the central portion of the polished workpiece surface to be raised or upwardly convexed, resulting in deteriorated flatness of the polished workpiece surface.
There is proposed another type of surface polishing method and apparatus wherein a plurality of workpieces are held on the work holding surface of a work holder plate adapted to be rotated about its axis, such that the workpieces are arranged around the axis of rotation of the work holder plate. In this type of surface polishing method and apparatus, the axis of rotation of the work holder plate does not lie within the workpiece surface to be polished, so that the problem of the central raised or convexed portion of the polished workpiece surface can be solved. However, the workpiece surface is subjected to a greater amount of stock removal by the abrasive grains at a local circumferential portion thereof which is relatively distant from the axis of rotation of the work holder plate, whereby the polished surface of each workpiece is inclined, resulting in deteriorated geometrical and dimensional accuracies of the polished surface. Further, this type of surface polishing method and apparatus requires the work holder plate which is as sufficiently large as about two or more times the diameter or size of the workpiece, whereby the required size of the polishing plate and that of the surface polishing apparatus are inevitably increased, leading to an increased cost of manufacture of the apparatus and an accordingly increased cost of polishing operation.